pull the lens & check with a test light. make sure you are getting juice when you are supposed to, where you are supposed to. if you are, then it is either another bad bulb or the pins are not aligned properly when it was inserted. if you have problems - pull the trim panels for the rear window washer reservoir and jack storage area and inspect the wiring (errant loading of jack can skin wires if harness is outta place). you should also pull out the fixture to make sure all of your connections are good at the sockets. you can check with the test light on the inside of the fixture too in case there is no power on socket side to isolate where problem is. sometimes this takes two people to do it more easily and check the reverse lights. if everything is good you have to trace your way back to the fuse looking for a break in the wire or what ever else could be it.
one of the PO's of my car hacked up the wiring in there to hook up his trailer hitch plug... he messed up some of the connections a little ways back behind the panel. when i initially checked at the sockets things were goofy and got no power on two of the sockets. when i pulled the fixture and stretched the wiring; the sockets had power. every thing is color coded and the wiring diagram is in the SM, haynes and Chilton's. make sure you are getting juice when you are supposed to.
you already know this one is not working properly, but: a good tip to see if all of your lights are working is to pull up to a store that has full height glass windows (easier to see things at nite but can do it during the day) and initiate all of your lighting sequences... parking, driving highs & lows, blinkers and flasher... then turn the car around and check in rearview same thing and reverse lights... when you are turning around scope out your side marker lights. easy way to check completely without getting in and out of the car and checking each bulb individually. great for lazy old people and beats being notified by law enforcement with a warning ticket.